Currently, mobile communication technologies are evolving towards higher frequency, larger carrier bandwidth, higher data rate and more heterogeneous layers. The future mobile network, e.g., the 5th generation (5G) mobile network, is likely to be a combination of the 3rd generation (3G) technologies, the 4th generation (4G) technologies and new technologies such as Ultra-Density Network (UDN). In order to meet the increasing demand on higher wireless capacity, the use of frequencies in tens of GHz range has been evaluated. In such evaluations, high frequency bands, for instance, in the frequencies of 10, 30, 60 and 98 GHz are used for the 5G mobile networks. At such frequencies, a very large bandwidth of spectrum is available. Both operating frequency and bandwidth of the 5G networks will be much higher than those used in the current mobile networks e.g., 3G or 4G networks.
However, larger signal attenuation or path loss comes along with higher operating frequency. Typically, a received power of a signal at a receiver can be expressed as:
                              P          rx                =                              P            tx                    ·                      G            tx                    ·                                                    G                rx                            ⁡                              (                                  λ                                      4                    ⁢                    π                    ⁢                                                                                  ⁢                    r                                                  )                                      2                    ·                      e                                          -                α                            ⁢                                                          ⁢              r                                                          (        1        )            where Ptx is the transmitted power of the signal, Gtx and Grx are gains of the transmit and receive antennas, respectively, λ is the wavelength of the signal, e is a constant value, α is an attenuation factor associated with absorption in the propagation medium, and r is the distance from the transmitter to the receiver. For example, for a millimeter wave link at 60 GHz, the parameter capturing the oxygen absorption loss can be up to16 dB/km.
It can be seen from Equation (1) that the attenuation of a radio wave is proportional to 1/λ2. For example, with the same propagation distance, the attenuation of a signal at 60 GHz will be 29.5 dB higher than that of a signal at 2 GHz without considering the oxygen absorption.
In order to compensate for the high attenuation at high frequency, high gain beamforming has been proposed. FIG. 1 shows an example of high gain beamforming. As shown in FIG. 1, an Access Node (AN) 110 has a number (8 in this case) of high gain beams (Beams #0˜#7) each covering an area referred to as sector. The AN may broadcast signals to terminal devices 120 and 122 in these sectors by means of beam sweeping.
Furthermore, at higher frequency, the ability for radio waves to penetrate through, or diffract at, blocking objects, such as buildings, vehicles and human bodies, becomes weaker. FIG. 2 shows an exemplary scenario where the terminal device 120 camping on one sector of the AN 110, i.e., the sector associated with Beam #0, loses its camping when it moves into a shadow area behind a wall. In this case, it will take relatively long time for the terminal device 120 to find another sector or AN to camp on, e.g., by blind searching. Such loss of camping may become more frequent due to blocking by human body. For example, the terminal device 122 camping on one sector of the AN 110, i.e., the sector associated with Beam #2, may lose its camping simply because its user blocks Beam #2.
In a mobile communication system, Track Area (TA) information for each registered terminal device is stored at a core network. When there is an incoming session or call for a terminal device, which may be in an idle state, the core network first finds out the TA of the terminal device and then sends a paging message to all ANs in the TA. Each AN that has received the paging message shall broadcast the paging message since the network may not be aware of which AN the terminal device is currently camping on. If high gain beamforming is adopted, the paging message may be broadcasted by means of beam sweeping. If the terminal device does not response to the paging message within a defined time period (e.g., due to loss of its camping AN as a result of blocking by a building or human body), the core network has to expand the TA iteratively and send the paging message in the expanded area. In this case, the paging overhead for the terminal device will be significantly increased.
There is thus a need for an improved camping mechanism for a terminal device.